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Retractable fabric shading

I'm trying to create a parametric retractable fabric shading family. The part I'm getting hung up on--the first part--is how to make a parametric parabola (or reasonable-looking approximation). Imagine holding a piece of fabric along two opposite edges... it droops down in the center in the shape of a parabola. Move the edges closer together and you it droops lower, pull them apart and it flattens out. How would you go about making this? Can I do it as a regular family? Does it need to be an adaptive component? What do you think?

Re: Retractable fabric shading

Well, first off, do you need it to be completely parametric or only show in its open or closed position?

Then comes the questions of are you willing to model the deflection caused by gravity, as the fabric won't keep a parabolic shape when in its closed (retracted) position, instead forming a teardrop shape, since you can't get the edges of a parabola to bow in towards the vertex.

That being said, it might be doable with a 3 point spline in an adaptive component. Using any other method requires too many other constraints and calculations.

Re: Retractable fabric shading

Excellent questions.

I would like it to be fully parametric. Why? Because (a) it is a challenge and (b) when we do visualization of these things, we often go for realism and I've already been asked to show it in a few positions. For getting this thing constructed, I wouldn't bother with this being parametric, but I think it will help my sanity during our design process.

It doesn't have to be accurate. I was thinking a parabola because it is a mathematical shape I could deal with more easily. That said, it could be a spline or a series of arcs, or whatever to approximate the shape. As long as it looks realistic in many positions of opened vs. closed, that's good enough. Attached is an adaptive component that starts to get somewhere. The formulas that control the "droop" are totally wrong and make for an unconvincing change. This may be a dead end, but I thought I'd share it.

Re: Retractable fabric shading

Originally Posted by damon.sidel

Excellent questions.

I would like it to be fully parametric. Why? Because (a) it is a challenge and (b) when we do visualization of these things, we often go for realism and I've already been asked to show it in a few positions. For getting this thing constructed, I wouldn't bother with this being parametric, but I think it will help my sanity during our design process.

It doesn't have to be accurate. I was thinking a parabola because it is a mathematical shape I could deal with more easily. That said, it could be a spline or a series of arcs, or whatever to approximate the shape. As long as it looks realistic in many positions of opened vs. closed, that's good enough. Attached is an adaptive component that starts to get somewhere. The formulas that control the "droop" are totally wrong and make for an unconvincing change. This may be a dead end, but I thought I'd share it.

I totally meant that this should be a 5-point spline, I swear.

See attached for what it would normally look like when the shade is retracted. I can't help you with adaptive components as I haven't had a reason to make any yet. Obviously gravity will play into this somewhat, but at a distance shouldn't matter. All you'd need from here is some random ratio between the spacing of the supports and the overall curtain drop. You'd want some maximum/minimum values to aid in the calculation effort.

Re: Retractable fabric shading

I've had a chance to get back to this family. andrewk, thank you for your input. I tried your family and it is not quite what I need, as I do need parametric control over it in a retracted position AND an open position. So I tried the adaptive components method. The tricky part was the set of formulas I needed to get a good approximation of the droop based on the arc length of the parabola (really a catenary, but a parabola works well enough). I decided to approximate it with a triangle so that 2 * hypotenuse is about the arc length, then I can determine the height and width from the two legs of the triangle. Attached is the pattern-based curtain panel I created that does what I'm talking about. And a few screen shots of it in the open and closed positions. It's OK, but will need a little more work as we develop the system.

Re: Retractable fabric shading

I never did mean for it to be a working family, just to show graphically what I was talking about for a 5-point spline giving the shape I thought you'd want. Besides, you never did mention what type of shade you were referring to, so I was going out on a limb there.

Re: Retractable fabric shading

Originally Posted by contact.andrewk968454

I was going out on a limb there.

No need to apologize or explain! I appreciate that you took a whack at it. I think you make a good point about what it could look like in a closed position. Maybe I'll refine my definition as I go to make it more realistic for my renderings. Thanks for the input.

Re: Retractable fabric shading

Sure! That's pretty close. I assume you got your formula to work? My triangle approximation worked well enough for me and flexes reasonably well. When my boss saw the images with the shading fully extended and almost fully retracted he said "What's this? [Referring to the retracted portion.] When fabric is closed like this it isn't perfect, it should have wrinkles and waves in it." Guess I can't win... at that point, I don't think having a better formula is going to help.